1. Field of the Invention
The present invention relates to a phase-lock-loop for controlling a voltage controlled oscillator and, in particular, to an autoranging phase-lock-loop circuit for controlling a range programmable voltage controlled oscillator.
2. Description of the Related Art
The approach used to control a voltage controlled oscillator (VCO) in a typical phase-lock-loop circuit is a feedback loop that measures a phase error between an input reference signal and a VCO output signal. In the typical approach, the feedback loop modifies a VCO input control voltage to drive the phase and frequency of the VCO output signal to match the phase and frequency of the input reference signal. This results in a one-to-one relationship between the VCO input control voltage and the frequency of the VCO output signal.
One major limitation to this approach is that in order to guarantee the dynamic range of the VCO relative to the input reference signal, the VCO must be designed to operate over a wide range of frequencies (as much as a 10X range of frequencies) to account for variations in the fabrication of the VCO as well as variations in the operating conditions of the circuit such as changes in the supply voltage and ambient temperature.
A second major limitation is that a wide-pull, high-gain VCO is more susceptible to noise and circuit coupling problems than a narrow-pull, low-gain VCO. However, a narrow-pull, low-gain VCO presents problems of its own. For example, the low-gain VCO lacks the dynamic range necessary to guarantee that the VCO will function over all variations in fabrication, temperature, and supply voltage.
One solution to this problem is to design a VCO that has multiple overlapping low-gain frequency ranges such that the loop will have sufficient dynamic range for variations in fabrication, temperature, and supply voltage. Since both supply and temperature conditions are environmentally controlled (continuously varying) and the fabrication variations are fixed (once a device has been manufactured, it does not change), these two variations can be treated differently.
The problem with this solution, however, is that in a typical phase-lock-loop circuit, the feedback loop will be able to match the phase and frequency of the input reference signal in only one low-gain frequency range because there is no mechanism for stepping the VCO from one low-gain frequency range to another to find the optimal VCO frequency range of operation for that specific device (accommodate the fabrication variations).
Several mechanisms for stepping from one low-gain frequency range to another are available, however, each mechanism has drawbacks that make it unacceptable. One approach is to measure the VCO at the factory to determine the optimum range of operation and then provide the information to the end user who must then program the VCO to the proper operating range.
This approach presents several drawbacks. First, current factory testing must be expanded to accommodate measurements of the optimum range with a resulting increase in time and cost. Second, the end user must program the VCO, resulting in additional time and cost to the end user.
Another approach is to design programmable fuses into the VCO. In this approach, the VCO characteristics are measured at the factory and then a testing device programs the VCO as is done with PROM devices. The drawback here is that the fabrication process is necessarily more complicated with lower yields, and resulting higher device production costs.
Thus, there is a need for a mechanism for stepping a multiple-range low-gain VCO from one low-gain frequency range to another which solves the above described problems.